Picosecond-scale Heterogeneous Melting of Metals at Extreme Non-equilibrium States

TL;DR

This work addresses how extreme electron–ion nonequilibrium states induced by ultrafast lasers drive melting in metals. It develops a two-temperature model coupled to deep neural network potentials (TTM-DPMD with ETD-NN) to capture hot-electron–modified potential energy surfaces and electronic pressure effects. The authors demonstrate that electronic pressure relaxation can trigger ultrafast heterogeneous melting and surface-initiated phase fronts in tungsten and gold, with distinct time-resolved X-ray diffraction signatures that differentiate nonthermal expansion from thermoelastic expansion. This hot-electron–mediated lattice destabilization appears to be a universal pathway for laser-induced structural transformations, offering new guidance for interpreting time-resolved experiments and steering laser–matter interactions.

Abstract

Extreme electron-ion non-equilibrium states, generated by ultrafast laser excitation, lead to melting processes that are fundamentally different from those under conventional thermal equilibrium and remain not fully understood. Through neural network-enhanced multiscale simulations of tungsten and gold nanofilms, we identify electronic pressure relaxation as critical to heterogeneous phase transformations. This nonthermal expansion generates a density decrease that enable surface-initiated melting far below equilibrium melting temperatures, creating electronic pressure-driven solid-liquid interface propagation at a high speed of 2500 m/s -- tenfold faster than that of thermal heterogeneous melting mechanisms. Simulated time-resolved X-ray diffraction signatures distinguish this nonthermal expansion from thermal expansion dynamics driven by thermoelastic stress. These results establish hot-electron-mediated lattice destabilization as a universal pathway for laser-induced structural transformations, providing new insights for interpreting time-resolved experiments and controlling laser-matter interactions.

Figures (10)

• Figure 1: Diverging melting dynamics in laser-excited tungsten. Under absorbed laser fluence of $120\ \rm{mJ\ cm^{-2}}$, (a) Conventional TTM-MD prediction with ground-state PES $A(\mathcal{R})$ showing homogeneous melting governed by electron-phonon coupling. (b) Our TTM-DPMD results with laser-excited PES $A (\mathcal{R}, T_{\rm e})$ revealing electronic pressure-driven heterogeneous melting. The local structures are identified by the polyhedral template matching (PTM) method larsen2016, and polyhedral surface meshes around FCC-type (green) and amorphous-type (gray) particles are constructed to highlight the heterogeneity in lattice symmetry. Red arrows indicate the propagation direction of phase transition (solid-liquid or BCC-FCC) interfaces. The atomic configurations are visualized by OVITO software ovito.
• Figure 1: Diverging melting dynamics in laser-excited tungsten. Under absorbed laser fluence of $120\ \rm{mJ\ cm^{-2}}$, (a) Conventional TTM-MD prediction with ground-state PES $A(\mathcal{R})$ showing homogeneous melting governed by electron-phonon coupling. (b) Our TTM-DPMD results with laser-excited PES $A (\mathcal{R}, T_{\rm e})$ revealing electronic pressure-driven heterogeneous melting. The local structures are identified by the polyhedral template matching (PTM) method larsen2016, and polyhedral surface meshes around FCC-type (green) and amorphous-type (gray) particles are constructed to highlight the heterogeneity in lattice symmetry. Red arrows indicate the propagation direction of phase transition (solid-liquid or BCC-FCC) interfaces. The atomic configurations are visualized by OVITO software ovito.
• Figure 2: Extreme heterogeneity in laser-excited W. (a,b) Depth-dependent thermodynamic pathways in tungsten nanofilm under laser fluence of $120\ \rm{mJ\ cm^{-2}}$. Colored circles show thermodynamic states at 0.5 ps intervals, with arrows indicating evolution trajectories. Red stars mark three representative thermodynamic states of surface region ($z=1.8\ \rm{nm}$) corresponding to: initial electronic pressure buildup ($t=0.5\ \rm{ps}$), complete pressure release ($t=1.5\ \rm{ps}$), and surface melting ($t=3\ \rm{ps}$). Blue stars denote simultaneous thermodynamic states of interior region ($z=14.2\ \rm{nm}$) for comparison. The calculated equilibrium isochore (gray solid line) and melting curve (black solid line) are also presented for comparison zeng2023. (c)(e) Radial distribution functions $g(r)$ and (d)(f) coordination numbers (CN) for surface and interior regions at selected times, where radial distribution of CN are obtained by integration of $g(r)$.
• Figure 2: Extreme heterogeneity in laser-excited W. (a,b) Depth-dependent thermodynamic pathways in tungsten nanofilm under laser fluence of $120\ \rm{mJ\ cm^{-2}}$. Colored circles show thermodynamic states at 0.5 ps intervals, with arrows indicating evolution trajectories. Red stars mark three representative thermodynamic states of surface region ($z=1.8\ \rm{nm}$) corresponding to: initial electronic pressure buildup ($t=0.5\ \rm{ps}$), complete pressure release ($t=1.5\ \rm{ps}$), and surface melting ($t=3\ \rm{ps}$). Blue stars denote simultaneous thermodynamic states of interior region ($z=14.2\ \rm{nm}$) for comparison. The calculated equilibrium isochore (gray solid line) and melting curve (black solid line) are also presented for comparison zeng2023. (c)(e) Radial distribution functions $g(r)$ and (d)(f) coordination numbers (CN) for surface and interior regions at selected times, where radial distribution of CN are obtained by integration of $g(r)$.
• Figure 3: Laser fluence dependence of structural transformation in W. (a) density decrease after electronic pressure relaxation along (100) direction, insets denote the uniaxially-distorted BCC, FCC with stacking faults, and FCC respectively. (b) isochoric and isobaric melting behavior under non-equilibrium condition, obtained from fixed-$T_{\rm e}$ DPMD simulations via two-phase method. The error bar associated with the melting point is defined as half the temperature interval between the two-phase method simulations where the solid phase is stable and those where it is molten. (c) complete melting time under different laser fluence. The yellow, red, and green region denotes the heterogeneous, homogeneous, and the ultrafast heterogeneous melting mechanism respectively. The error bar for the complete melting time is defined as the temporal resolution of the atomic trajectory output from the TTM-DPMD simulations.